1. Field of the Invention
The present invention relates to a semiconductor device formed by using a semiconductor film having a crystalline structure, and to a method of manufacturing the semiconductor device. In particular, the present invention relates to a semiconductor device containing a field effect transistor in which a channel forming region is formed by a crystalline semiconductor film formed on an insulating surface, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
Techniques of forming an amorphous silicon film on an insulating substrate such as glass, crystallizing the amorphous silicon film, and then forming semiconductor elements such as transistors have been developed. In particular, techniques of crystallizing amorphous silicon films by irradiating laser light have been applied to techniques of manufacturing thin film transistors (TFTs). Transistors manufactured by using a semiconductor film having a crystalline structure (crystalline semiconductor film) are applied to liquid crystal display devices, typically flat surface type display devices (flat panel displays).
The application of laser light in semiconductor manufacturing processes has developed in techniques of recrystallizing damaged layers and amorphous layers formed in a semiconductor substrate or a semiconductor film, and techniques of crystallizing an amorphous semiconductor film formed on an insulating surface. A suitable laser oscillator apparatuses normally use a gas laser, typically an excimer laser, or a solid state laser, typically a YAG laser.
An example of crystallizing an amorphous semiconductor film by laser light irradiation is polycrystallization as disclosed in JP 62-104117 A, in which the laser light scanning speed is at high speed, equal to or greater than the beam spot size×5000/sec, and the amorphous semiconductor film thus does not reach a completely melted state. A technique of effectively forming a single crystal region by irradiating elongated laser light to a semiconductor film formed into an island shape is disclosed in U.S. Pat. No. 4,330,363. Alternatively, a method is known for processing laser light into a linear shape beam by using an optical system and then irradiating the beam, as in a laser processing apparatus disclosed in JP 8-195357 A.
In addition, a technique for manufacturing a transistor by irradiating the second harmonic of laser light, emitted using a solid state laser apparatus such as an Nd:YBO4 laser, to an amorphous semiconductor film, thus forming a crystalline semiconductor film having a large grain size compared to conventional crystalline semiconductor films, is disclosed in JP 2001-144027 A.
However, polycrystals result if crystallization is performed by irradiating laser light to an amorphous semiconductor film formed on a level surface, defects such as crystal grain boundaries are formed arbitrarily, and crystals with aligned orientation cannot be obtained.
Grain boundaries are included as crystal defects, and the crystal grain boundaries become carrier traps, and causes for lowering the mobility of electrons or holes. Furthermore, semiconductor films in which distortions and crystal defects do not exist cannot be formed because of volumetric contraction of the semiconductor film, thermal stress with a base, lattice mismatching, and the like that accompany upon crystallization. Unless otherwise using a particular method in which a laminated SOI (silicon on insulator) is used, it has not been possible to obtain a quality equivalent to that of a MOS transistor, which is formed on a single crystal substrate, in a crystalline semiconductor film that is formed on an insulating surface and then crystallized or recrystallized.
The aforementioned flat panel display devices and the like have built-in transistors in which a semiconductor film is formed on a glass substrate, but it is nearly impossible to dispose the transistors so as to avoid arbitrarily formed grain boundaries. That is, unintentionally included crystal grain boundaries and crystal defects have not been able to be excluded by strictly controlling the crystallinity of transistor channel forming regions. In short, not only do the electrical characteristics of the transistor degrade, but also this becomes a cause of dispersion in the characteristics of individual elements.